A Soft-Switching Isolated Three-Port Converter for Microgrid Integration of SPV Source and Auxiliary Battery Charging Using Transformer Leakage Energy

Abstract

This paper presents a soft-switching isolated three-port DC-DC converter that interfaces a solar photovoltaic (SPV) source and a battery storage unit with a DC microgrid or local load. The proposed topology adopts a flyback converter structure augmented with a simple active auxiliary circuit that recovers transformer leakage energy and enables soft commutation of the semiconductor devices. Distinctively, the recovered leakage energy is directly utilized to charge a 3.7-V Li-ion battery, introducing a novel method of harnessing leakage energy for battery charging, which has not been reported in prior literature. The battery port, connected in series with the SPV source, enhances the converter's effective voltage gain while simultaneously minimizing the transformer leakage flux. Moreover, the proposed converter achieves reduced losses with a lower component count by employing soft-switching techniques, thereby improving overall power conversion efficiency and power density. The switching devices operate under ZV–zero-current switching (ZCS) turn-on and zero-voltage switching (ZVS) turn-off (main switch), ZCS turn-on, and ZV–ZCS turn-off (auxiliary switch), while the diodes exhibit soft recovery characteristics. Furthermore, a firefly algorithm-based MPPT enhanced with incremental conductance refinement is implemented to improve the tracking accuracy and dynamic response of solar power extraction. Comprehensive operational analysis, design methodology, and steady-state gain evaluation are presented. The proposed converter's effectiveness is validated experimentally using a 100-W laboratory prototype controlled by an STM32 Nucleo-G474RE microcontroller for MPPT and output voltage regulation.